The present invention is in the field of electrical power supplies and, more particularly, switched-mode power supplies (SMPS's) with uncoupled output inductors.
It is well known that a multioutput SMPS with output inductors may experience an overvoltage condition at an inductor output if a load of the SMPS drops below a minimum level at that output. This loading condition may be referred to as a “low-load” condition. In contemplation of this phenomenon, many prior-art SMPS's employ coupled inductors as output inductors for multiple loads. In such an arrangement, development of a “low-load” condition in one load may be offset with a suitably high coupled load. Thus, the coupled inductors may be provided with correction by a regulation control loop when one of the loads of the SMPS reduces. This may occur even if the total load of the SMPS reduces to a state in which any one of the inductors may go into discontinuous conductance. Over-voltage conditions may be thus avoided.
In some application of an SMPS, a common converter may be used to power two or more different loads for which coupled inductors are not suitable. For example, one load may be a logic circuit and another load may be a motor load. It is desirable, in such a case, to de-couple the load effects of the motor load from the logic circuit. In this context, an SMPS may be configured so that a separate output inductor may be provided for each of the loads. In such an arrangement special provisions must be made to accommodate the possibility that one or more of the output inductors may be exposed to a “low-load” condition. A low-load condition may produce an excessive voltage or “overvoltage” at the output.
In prior-art SMPS's, this overvoltage has been prevented through use of bleeders or post voltage regulators. A typical prior-art bleeder may consist of a power dissipation resistor connected in parallel with a load of the SMPS. The power dissipation resistor will continuously conduct current and dissipate energy, even if the nominal load is maintained above a “low-load” condition. Presence of this resistive load may preclude development of excessive voltage at the output even if the nominal load decreases to a “low-load” state.
In order to be effective, a resistive bleeder must produce a current high enough to preclude overvoltage development at the output. In that regard, the resistor must continuously consume electrical power and dissipate thermal energy. Consequently, such prior-art bleeders may contribute to reduced efficiency of an SMPS. Additionally, dedicated cooling of the bleeder may be required in order to continuously dissipate thermal energy from the bleeder resistor.
In an alternate prior-art scheme of overvoltage control, a voltage regulator may be placed in series with the nominal load. The voltage regulator will drop the excessive voltage developed to ensure regulation and the right voltage at the output when the nominal load drops to a low-load condition. But, as with the bleeder described above, the voltage regulator must operate continuously even when the nominal load is present. The voltage regulator needs to drop some voltage across itself for proper functioning. This introduces unwanted power dissipation and decreases the efficiency of the SMPS.
As can be seen, there is a need to provide efficient overvoltage control during “low-load” conditions for an SMPS. In particular, there is a need to provide such overvoltage control in an SMPS without using coupled inductors or continuous operation of bleeders or post voltage regulators.